What are the potential benefits of polyandry for female animals?

Question

What are the potential benefits of extra pair copulations for female animals?

Answer

A mating system in which females mate with multiple males is known as polyandry. Although there are many animals in which this is the norm, it is also thought to frequently occur within species which are generally considered monogamous, including many species of bird. Extra-pair copulations are thought to be highest among fairy wrens, tiny birds endemic to Australia, in which over 65% of chicks are fathered by males outside of the breeding pair.

There are a variety of potential benefits to the females in this scenario. Competition between sperm cells should ensure that the majority of the offspring are the progeny of the “highest quality” sperm, which should correlate with high offspring fitness, although evidence as to whether this is the case is limited. It also increases the genetic variation among the offspring, increasing the likelihood of offspring survival in difficult circumstances.

In species in which the males help care for the young, extra pair copulations can gain a female extra parental care for their offspring. This has been observed in several species of birds, mammals, and one family of frogs. As the males are unable to tell which of the offspring are their own, they will help care for all of the young, increasing their chances of survival. Similarly, mating with multiple males can also confer protection from infanticide in species where this is common, which includes many mammals. Infanticide generally seems to be motivated by an urge to reduce the competition with the individual’s own offspring, or to relieve competition for resources. Males are generally much less likely to attempt this if they might be related to the offspring in question, and may step in to defend ‘their’ young from other males.

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